In the field of medicine and clinical chemistry, carriers such as magnetic or non-magnetic particles, beads, tubes, wells, microtitration plates, strips and sheets, are often used in vitro, e.g., in specific binding assays as a solid support and/or as a label, for affinity purification of substances, for separation of cells, or as enzyme carriers in enzymatic processes; or in vivo, e.g., as drug delivery systems, for the localization of disease sites or for measuring the blood flow in an organ. Usually, the carriers are associated with one or more specific binding pair partners.
Some of the difficulties associated with use of the carriers are (1) attaching specific binding partners to the carrier and, (2) preventing non-specific binding of undesired substances to the carrier from samples brought into contact with the carrier. Non-specific binding is the result of non-covalent binding between molecules that is relatively independent of specific surface structures. Non-specific binding may result from several factors including hydrophobic interactions between molecules.
The consequences of these problems include an unacceptably low number of specific binding partners associated to the carrier, poor performance of specific binding assays (high background and low sensitivity), and low purity in affinity purifications. Particular problems exist for carriers in the shape of particles which may agglutinate unspecifically and, in consequence, may form undesirable precipitates of aggregated particles.
Solutions to these problems have been approached through the use of carriers with hydrophilic polysaccharide coatings. In WO 90/04178 an immunoreactant carrier is described wherein a polysaccharide layer is coated to the surface of the carrier by covalent binding. The carrier described in WO 84/03358 is characterized by a surface which is only partially coated with a polysaccharide and elsewhere is not covered by such a coating but instead a specific binding partner is attached. The use of magnetic particles as pharmaceutical agents is described in WO 96/27394, wherein these particles are coated by alkali-treated polysaccharides. Iron-containing nanoparticles comprising of an iron-containing core and two polymer coatings are known from WO 96/04017. Squarate dyed carboxylate modified latex particles can be coated by maleimidated aminodextran according to EP 0 275 139.
Particles having multiple coating of aminodextrans are described in U.S. Pat. No. 5,639,620 and U.S. Pat. No. 5,707,877. Here, it is described that the multiple aminodextrans layers may be crosslinked through the use of a bifunctional crosslinking agents such as gluteraldehyde. The attachment of a biological substance to free amine groups on the outer layer of dextran also involves the use of the crosslinking agent. While this procedure allows for particles with multiple coatings of an amino dextran which should therefore diminish non-specific binding, the particles preparation and attachment to biological substances involve the use of the crosslinking reagent. This adds an extra step during the synthesis of the carrier or the attachment of biological substances. Further, the reactions involving the crosslinking reagent must be carefully controlled to avoid intra-layer and inter-particle crosslinking. Accordingly, it is still desirable to have a carrier where all of the surface is covered with a hydrophilic coating which is easily prepared and to which specific binding pair partners may be easily attached.